Abstract
Compass/Beidou2 uses uniform GPS-type broadcast ephemerides for its mixed constellation, which consists of geostationary earth orbit (GEO), inclined geosynchronous orbit and medium earth orbit satellites. The ephemeris parameters adopted by Compass are singular for the small inclinations of GEOs, and some intermediate orbital planes are currently used to avoid the singularities of parameter fitting. To directly deal with GEOs, an improved 18-element broadcast ephemeris is proposed, which consists of the non-singular elements and some correction terms. First, the non-singular elements are used to enable the orbit representation of GEOs to be completely free from the singularity. Second, two additional parameters, which express the predominant characteristics of east---west drift of the subsatellite point of GEO, are added into the correction terms for improving the modeling precision. Third, the user satellite position and velocity computation model based on the designed broadcast ephemeris parameters are presented. Simulation results show that the fitting arc length of the proposed broadcast ephemeris can be reduced to as short as 2 h, and that the improvement of the fitting user range error (URE) is obtained. The typical RMS of the fitting URE obtained in a non-eclipse period is less than 1 and 5 mm for 2- and 3-h fitting arcs, respectively.
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